1. Field of the Invention
The present invention relates to a vapor-phase growth method in which a thin film is formed on the surfaces of semiconductor substrates, and which is employed in the process of manufacturing semiconductor devices such as VLSI (Very Large Scale Integration) devices.
2. Description of the Related Art
A CVD (Chemical vapor Deposition) method used in manufacturing semiconductor devices such as VLSI devices is a method wherein a thin film is formed on a semiconductor substrate, utilizing chemical reaction proceeding at the surface of the substrate or in the vapor phase over the surface of the substrate. The method finds its main use in forming an insulating film such as a silicon oxide film or a silicon nitride film.
A thin-film deposition technique, which forms a thin film with good step coverage, on the surface of a semiconductor substrate and in trenches formed in the surface thereof and having a high aspect ratio (a depth-to-width ratio), is becoming indispensable to the manufacture of recently developed VLSI devices. The conventional CVD method (e.g., the one disclosed in J. L. Vossen & W. Kern, Thin Film Process, Academic Press, 1978) may be employed to form, as shown in FIG. 1, a silicon oxide film 100 in a trench 104a made in the surface of a silicon substrate 104. When it is employed for this purpose, however, the species formed in the vapor phase is deposited more on an edge of the trench 104a than on any other part thereof. This makes it increasingly difficult for the species to deposit on the bottom of the trench 104a. Consequently, a hole is left in the trench 104a, and the resultant thin film will have but poor step coverage.
Where a silicon oxide film is formed by a method of thermally decomposing TEOS (Tetraethoxysilane) (e.g., the method disclosed in R. D. Rang, Y. Momose & Y. Nagkubo, IEDM. TECH. DIG., 1982, p. 237), a deposition temperature must be at least 600.degree. C. At so high a temperature as this, it is difficult to form a thin film on an aluminum wiring having a low melting temperature.
According to TEOS plasma CVD, it is possible to deposit SiO.sub.2 at a low temperature. In this method, however, since TEOS decomposes excessively in plasma, it is difficult to form a SiO.sub.2 film having a good step coverage.
Another method of forming a thin film is known, in which use is made of reaction between TEOS and ozone. This method can indeed provide fairly good step coverage. The film formed by the method is not sufficiently insulating since ozone is not so reactive. Inevitably, any device having a thin film formed by this method has but a short lifetime.
As described above, with the conventional methods it is difficult to form a highly insulating thin film exhibiting good step coverage, on the surface of a semiconductor substrate and in a trench formed in the surface thereof and having a high aspect ratio.